This invention relates to a matrix display device comprising a plurality of display elements arranged in a matrix and defined by respective electrodes carried on opposed substrates between which is disposed an electrically excitable display material which, together with the electrodes, exhibits capacitance in operation, and in which the display elements are controlled by selectively applying voltage to the respective electrodes via respective switching means carried on one substrate.
In a known example of such a display device in which the electrically excitable display material comprises liquid crystal material, each display element is defined by a respective electrode carried on the one substrate and an opposing portion of an electrode common to all display elements carried on the other substrate. The switching means comprise thin film transistors (TFTs) and are arranged in an array of rows and columns. A plurality of data lines are each connected to a respective column of the TFTs and a plurality of gate lines are each connected to a respective row of the TFTS.
The device is driven by repetitively scanning the gate lines in sequential fashion so as to turn on all TFTs in each row in turn and applying picture (data) signals to the data lines for each row in turn so as to build up a display picture. When each line of TFTs is turned "on", the TFTs in that line form conductive paths between the data lines and the one electrode of each of the display elements of that line which thus charge up to the applied data voltage. When the gate line voltage falls at the termination of energisation of that line, the TFTs in that line are all turned "off". This isolates the display element electrodes and charge is stored on each display element in view of the inherent capacitance of the display element structure although storage capacitors may also be used. The display elements stay in the state into which they were driven, determined by the applied data voltage, until the next time the associated TFTs are turned on, in the case of TV signals, this being every field period.
This kind of display device may be used for data graphic purposes or to display TV pictures by sampling each line in the video signal and placing the appropriate voltages on the data lines.
Particularly when the display device is being used to display TV pictures, a grey scale capability is needed. In known devices this is achieved in practice utilising the display element's transmission/voltage characteristic and by adjusting the voltage applied across each display element accordingly. The liquid crystal material employed is chosen to have a voltage switching characteristic that exhibits an exponential slope and varies slowly with voltage applied thereacross. This necessary restriction on the kind of liquid crystal matter used to one having a gradual switching characteristic also results in limitations of other properties of the material such as viewing angle in particular.